Due to the low outside temperatures and the low atmospheric pressure at high altitudes, environmental control systems are an indispensable component of modern commercial aircraft. Only the utilization of such systems makes it possible to transport passengers over greater distances. The architecture of environmental control systems is subject to a constant adaptation and optimization process. For example, new regulations stipulate that aircraft be equipped with an inerting system for generating and introducing nitrogen into the fuel tanks for safety reasons in order to reduce the fuel explosion hazard. Such inerting systems are also known under the English technical term “On Board Inert Gas Generation Systems” (abbreviated “OBIGGS”). In the context of the optimization of environmental control systems, the additional fuel consumption required for the operation likewise needs to be minimized during their design.
Environmental control systems that operate with bleed air diverted from engines are currently the most widely used design. In such environmental control systems, the bleed air is usually diverted from one or more engines at one of two different locations depending on the flight situation, e.g., an intermediate pressure opening (“intermediate pressure port”) and a high pressure opening (“high pressure port”). The utilization of bleed air for the air-conditioning is advantageous because the bleed air has a relatively high pressure, as well as a relatively high temperature, and therefore can be regulated to a broad range of desired pressures and temperatures. Bleed air diverted in the described fashion can also be used for other consumer systems such as, for example, de-icing systems (“wing ice protection systems”) or even inerting systems.
Alternatively, inerting systems may also be supplied with used air that is extracted from the passenger cabin of the aircraft. The air in a passenger cabin is regularly renewed by supplying the cabin with fresh air and discharging existing cabin air therefrom. A significant portion of the air discharged from the cabin is reused by admixing it to the flow of fresh air being supplied to the cabin. The ratio of fresh air to recirculated air typically amounts to approximately 50-60%. The remainder of the cabin air is discharged into the surroundings of the aircraft, wherein outflow valves provided for this purpose may also be simultaneously used for regulating the cabin pressure.
An inerting system usually comprises an air separation module that contains, for example, zeolite membranes, through which an air flow is pressed. Due to the different mass transfer rates for nitrogen and oxygen, this makes it possible to split the air flow such that air flows with higher nitrogen content and higher oxygen content are obtained. The air fraction enriched with nitrogen is routed into fuel tanks such that the oxygen present at this location is displaced. The air fraction enriched with oxygen is frequently not reused or routed into the passenger cabin after it is processed with elaborate auxiliary measures.
The efficiency of an inerting system essentially depends on the ratio of input pressure to output pressure, on the relative humidity of the air, on the temperature, the air quality and the ozone concentration. In order to optimize the air separation process and to protect the sensitive membranes from damages, the inerting system usually conditions the bleed air before it reaches the membranes. The devices required for this process such as compressors, filters, air cooling modules, waters separators and the like are integrated into the inerting system and create corresponding costs, weight and space requirement.
For example, U.S. Pat. No. 7,172,156 B1 describes an inerting system that is supplied with bleed air from aircraft engines. The bleed air is pre-conditioned with filters and heat exchangers before it reaches the air separation module. The air fraction enriched with nitrogen is routed to fuel tanks and the air fraction enriched with oxygen is discharged outboard into the surroundings of the aircraft. The disclosed inerting system furthermore contains a compressor and a turbine that lead to a high complexity of the inerting system, as well as a high weight thereof.
US 2007/0062371 A1 describes an inerting system that is supplied with used cabin air, wherein the air fraction enriched with oxygen is returned into the cabin from the inerting system. However, this publication does not elaborate on how a pressure differential between the input air flow and output air flows, which is required for the function of the inerting system, should be generated during all flight phases. At the cruising altitude, the air enriched with oxygen is discharged into the atmosphere such that it expands. An air inlet is simultaneously positioned in the immediate vicinity of the air outlet and once again takes in the air enriched with oxygen that was discharged into the surroundings in order to compress it to a higher pressure than the pressure in the passenger cabin with the aid of compressors. However, mixing with ambient air takes place during this process.
Lastly, EP 0975518 B1 and EP 1358911 B1 describe an environmental control system, in which an oxygen generating device (“On Board Oxygen Generation System” or, in abbreviated form, “OBOGS”) is used for enhancing the comfort by introducing air with an elevated oxygen content into the cabin.